Sputter deposition is one of the most widely used techniques for the fabrication of metal or metal oxide thin-film structures on substrates such as semiconductor wafers or glass or plastics. Sputter deposition is the deposition of particles vaporized from a surface (sputter target) by the physical sputtering process. A review of sputter deposition process is available entitled “Sputter Deposition For Semiconductor Manufacturing” by S. M. Rossnagel in IBM J. of Research & Development, 43, 163 (1999). Physical sputtering is a non-thermal vaporization process where surface atoms are physically ejected by momentum transfer from an energetic bombarding particle that is usually a gaseous ion accelerated from plasma or an “ion gun.” A plasma is a gaseous environment where there are enough ions and electrons for there to be appreciable electrical conductivity. Vacuum deposition is the deposition of a film or coating in a vacuum (or low-pressure plasma) environment. Generally, the term is applied to processes that deposit atoms or molecules one at a time.
Sputter deposition is usually carried out in diode plasma systems known as magnetrons. It is usually performed in a vacuum or low-pressure gas (<5 mTorr) where the sputtered particles do not suffer gas-phase collisions in the space between the target and the substrate. It can also be done in a higher gas pressure (5-15 mTorr) where energetic particles that are sputtered or reflected from the sputtering target are “thermalized” by gas-phase collisions before they reach the substrate.
Cryogenic solids are solids that only form at very low temperatures. Cleaning of surfaces utilizing impingement of cryogenic solid particles of relatively inert gases such as argon and CO2 are known. The size of the material removed is much larger than in physical sputtering. An example of a conventional carbon dioxide cleaning system is described in U.S. Pat. No. 5,766,061 entitled “Wafer Cassette Cleaning Using Carbon Dioxide Jet Spray” by Charles W. Bowers. More recent examples of methods and apparatus employing the process are found in U.S. 20050263170 entitled “Methods For Resist Stripping And Other Processes For Cleaning Surfaces Substantially Free Of Contaminants” by A. Tannous and K. Makhamreh and U.S. 20050272347 entitled “Dry Ice Blasting Cleaning Apparatus” by B. Spalteholz and G. Nielsen.
U.S. 20050263170 suggests that the combination of sublimation of a spray of cryogenic solid particles as they impinge the surface to be cleaned, and the impact momentum transfer by the particles, provide the vehicle for removing contamination from a surface. It further suggests that sublimation occurs, and therefore a major portion of the cleaning, only while the surface to be cleaned is at a higher temperature than that of the cryogenic solid spray. The thermophoresis due to the heated surface also helps to remove the particles from the surface and reduce the chance for re-deposition of the detached particles. As a consequence, heating of the surface being cleaned preferably is required within the vicinity of the impinging cleaning spray.
Carbon dioxide snow lift-off process has also made sputter metal process more reliable in semiconductor wafer manufacturing. U.S. Pat. No. 6,500,758 entitled “Method For Selective Metal Film Layer Removal Using Carbon Dioxide Jet Spray” by Charles W. Bowers describes a method for selective metal film layer removal using carbon dioxide jet spray. The CO2 snow lift-off removes the unwanted metal from the wafer by initiating a thermal expansion mismatch between the metal layer and the photoresist. This causes the metal to crack and delaminate at the photoresist interface. The loosened metal is then carried off the wafer surface by the CO2 snow.
U.S. Pat. No. 6,630,121 entitled “Supercritical Fluid-Assisted Nebulization And Bubble Drying” by Sievers et al. describes a method of making fine particles of a substance by forming a composition comprising a substance of interest and a supercritical or near supercritical fluid; rapidly reducing the pressure on said composition, whereby droplets are formed; and passing said droplets through a flow of heated gas. This approach requires the mixing of the substance to be made into fine particles with a high-pressure fluid and further use of a heated gas after the pressure is rapidly reduced. It would be desirable to enable controlled particle formation of a target material without the need of mixing the target material with a supercritical or near supercritical fluid in a high-pressure chamber.
Spray coating is another well-known process that involves passing a coating formulation under pressure through an orifice in to air in order to form a liquid spray that impacts a substrate and forms a liquid coating on the substrate. An example of spray coating is provided in U.S. Pat. No. 5,203,843, entitled “Liquid Spray Application of Coatings With Supercritical Fluids As Diluents And Spraying From An Orifice” by Kenneth L. Hoy et al. Hoy et al. describe a process of forming a liquid mixture in a closed system, said liquid mixture comprising at least one polymeric component capable of forming a coating on a substrate and a solvent component containing at least one supercritical fluid and spraying said liquid mixture onto a substrate to form a liquid coating thereon, by passing the mixture under pressure through an orifice into the environment of the substrate to form a liquid spray. This approach also requires the mixing under high-pressure which is undesirable.
It would also be desirable to enable controlled particle formation of a target material, and formation of films and dispersions from such particles, without the need for low-pressure systems such as employed in conventional sputter deposition systems. Use of cryogenic particles to controllably make fine particles, dispersions, or films from a target material has not been reported to date.